Vertical-cavity surface-emitting laser diode and optical transmission apparatus

ABSTRACT

A vertical-cavity surface-emitting laser diode includes: a first resonator that has a plurality of semiconductor layers comprising a first current narrowing structure having a first conductive region and a first non-conductor region; a first electrode that supplies electric power to drive the first resonator; a second resonator that has a plurality of semiconductor layers comprising a second current narrowing structure having a second conductive region and a second non-conductive region and that is formed side by side with the first resonator, the second current narrowing structure being formed in same current narrowing layer as the layer where the first current narrowing structure is formed; and a coupling portion as defined herein; and an equivalent refractive index of the coupling portion is smaller than an equivalent refractive index of each of the first resonator and the second resonator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/452,053 filed Aug. 5, 2014, which is based on and claims priorityunder 35 USC 119 from Japanese Patent Application No. 2013-163757 filedon Aug. 7, 2013. The entire disclosures of the prior applications areconsidered part of the disclosure of the accompanying continuation, andare hereby incorporated by reference.

BACKGROUND

1. Technical Field

The present invention relates to a vertical-cavity surface-emittinglaser diode and an optical transmission apparatus.

2. Related Art

Recently, a light source capable of providing transmission at a highrate up to about 100 Gb/s with low power consumption is required withthe accelerated increase of optical link transmission capacity. In orderto use a vertical-cavity surface-emitting laser diode as such a lightsource, the modulation rate of the vertical-cavity surface-emittinglaser diode must be further increased.

SUMMARY

According to an aspect of the invention, there is provided avertical-cavity surface-emitting laser diode including: a firstresonator which has a plurality of semiconductor layers including afirst current narrowing structure having a first conductive region and afirst non-conductor region; a first electrode which supplies electricpower to drive the first resonator; a second resonator which has aplurality of semiconductor layers including a second current narrowingstructure having a second conductive region and a second non-conductiveregion and which is formed side by side with the first resonator, thesecond current narrowing structure being formed in the same currentnarrowing layer as the layer where the first current narrowing structureis formed; and a coupling portion which couples the plurality ofsemiconductor layers in the first resonator with the plurality ofsemiconductor layers in the second resonator respectively; wherein: theequivalent refractive index of the coupling portion is smaller than theequivalent refractive index of each of the first resonator and thesecond resonator.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will be described indetail based on the following figures, wherein:

FIG. 1 is a schematic plan view of a vertical-cavity surface-emittinglaser diode according to a first Example of the invention and asectional view of the same vertical-cavity surface-emitting laser diodetaken along the line A-A of the plan view;

FIG. 2 is a view showing an equivalent refractive index and a lightconfinement distribution of the vertical-cavity surface-emitting laserdiode according to the first Example of the invention;

FIG. 3 is a graph showing frequency response characteristic of thevertical-cavity surface-emitting laser diode according to the firstExample of the invention;

FIG. 4 is a graph showing the relation between coupling efficiency andmodulation sensitivity of the vertical-cavity surface-emitting laserdiode according to the first Example of the invention;

FIG. 5 is a graph showing the relation among a coupling width, couplingefficiency and scattering loss of the vertical-cavity surface-emittinglaser diode according to the first Example of the invention;

FIG. 6 is a graph showing the relation between a delay time and amodulation bandwidth of the vertical-cavity surface-emitting laser diodeaccording to the first Example of the invention and the relation betweenreverse phase coupling and same phase coupling and the modulationbandwidth;

FIG. 7 is a schematic plan view of a vertical-cavity surface-emittinglaser diode according to a second Example of the invention and asectional view of the same vertical-cavity surface-emitting laser diodetaken along the line A-A of the plan view;

FIG. 8 is a schematic plan view of a vertical-cavity surface-emittinglaser diode according to a third Example of the invention and asectional view of the same vertical-cavity surface-emitting laser diodetaken along the line A-A of the plan view; and

FIG. 9 is a sectional view showing one example of the aspect of anoptical transmission apparatus using the vertical-cavitysurface-emitting laser diode according to any one of these Examples ofthe invention.

REFERENCE SIGNS LIST

10: VCSEL

20: drive mesa

30: coupling portion

32: constricted shape

40: control mesa

100: GaAs substrate

102: lower DBR

104: active region

106: current narrowing layer

106A: oxidized region

106B: non-oxidized region

106C: boundary

108: upper DBR

110: drive electrode

120, 122: control electrode

130: n-side electrode

200: insulated region

DETAILED DESCRIPTION

VCSELs (Vertical-Cavity Surface-Emitting Laser diode, hereinafterreferred to as VCSEL) according to exemplary embodiments of theinvention will be described below with reference to the drawings. Inaddition, the scale in each drawing is emphasized in order to make iteasy to understand features of the invention. Accordingly, it should benoted that the scale in each drawing is not always identical to thescale of a real device.

FIG. 1 includes a schematic plan view of a VCSEL according to a firstExample of the invention and a sectional view of the same VCSEL takenalong the line A-A of the plan view. As shown in (A) of FIG. 1, theVCSEL 10 according to the Example is configured to include a firstcolumnar structure 20, a coupling portion 30 and a second columnarstructure 40 which are formed monolithically on a substrate. The firstcolumnar structure 20 and the second columnar structure 40 are disposedin an X direction. The first columnar structure 20 is coupled to thesecond columnar structure 40 by the coupling portion 30. The firstcolumnar structure 20 serves as a drive mesa and includes a firstresonator. The second columnar structure 40 serves as a control mesa andincludes a second resonator. The coupling portion 30 includessemiconductor layers shared with the first columnar structure 20 and thesecond columnar structure 40. The coupling portion 30 at least opticallycouples the first columnar structure 20 and the second columnarstructure 40 to each other. In a preferable mode, the coupling portion30 serves for propagating a part of light generated by the firstcolumnar structure 20 toward the second columnar structure 40 andfeeding the light reflected on the second columnar structure 40 back tothe first columnar structure portion 20.

In (A) of FIG. 1, the first and second columnar structures 20 and 40 areformed substantially symmetrically in the X direction with respect tothe coupling portion 30 and the first and second columnar structures 20and 40 are formed into rectangular shapes in plan view. This is simplyan example. The first and second columnar structures 20 and 40 are notnecessarily symmetric. The first and second columnar structures 20 and40 are not limited to the rectangular shapes. Alternatively, the firstand second columnar structures may be cylindrical structures or columnarstructures formed into elliptical shapes in plan view. The first andsecond columnar structures may be formed into asymmetric shapes.Further, the first and second columnar structures may have differentshapes and different sizes from each other. Incidentally, in thefollowing description, the first columnar structure 20 will be referredto as drive mesa and the second columnar structure 40 will be referredto as control mesa for convenience's sake.

As shown in (B) of FIG. 1, the laminate structure of the VCSEL 10 is thesame as a typical 980-nm InGaAs/GaAs triple quantum well structure. Thatis, the laminate structure of the VCSEL 10 is formed out of a laminateof an n-type lower distributed bragg reflector 102 (hereinafter referredto as DBR), an active region 104 and a p-type upper DBR 108 on an n-typeGaAs substrate 100. The n-type lower DBR 102 has AlGaAs layers withdifferent Al compositions stacked alternately. The active region 104 isformed on the lower DBR 102 and includes a quantum well layer interposedbetween an upper spacer layer and a lower spacer layer. The p-type upperDBR 108 is formed on the active region 104 and has AlGaAs layers withdifferent Al compositions stacked alternately. The n-type lower DBR 102has a laminate of high refractive index layers and low refractive indexlayers, such as a laminate of plural pairs of Al_(0.92)Ga_(0.08)Aslayers and Al_(0.16)Ga_(0.84)As layers. The thickness of each of theselayers is λ/4n_(r) (λ designates an oscillation wavelength and n_(r)designates the refractive index of a medium). These layers are stackedalternately by 40 cycles. The carrier concentration after silicon as ann-type impurity is doped is, for example, 3×10¹⁸ cm⁻³.

In the active region 104, the lower spacer layer consists of an undopedAl_(0.3)Ga_(0.7)As layer, the quantum well active layer consists of anundoped In_(0.2)Ga_(0.8)As quantum well layer and an undoped GaAsbarrier layer, and the upper space layer consists of an undopedAl_(0.3)Ga_(0.7)As layer.

The p-type upper DBR 108 has a laminate of high refractive index layersand low refractive index layers, such as a laminate of plural pairs ofAl_(0.92)Ga_(0.08)As layers and Al_(0.16)Ga_(0.84)As layers. Thethickness of each of these layers is λ/4n_(r). These layers are stackedalternately by 25 cycles. The carrier concentration after carbon as ap-type impurity is doped is, for example, 3×10¹⁸ cm⁻³. A currentnarrowing layer 106 consisting of a p-type Al_(0.98)Ga_(0.02)As layer(or AlAs layer) is formed on or inside the lowermost layer of the upperDBR 108. In addition, a contact layer (for example, 1×10¹⁹ cm⁻³) made ofp-type GaAs with a high concentration of impurities may be formed on theuppermost layer of the upper DBR 108.

The current narrowing layer 106 has a higher Al composition than that ofeach of the lower DBR 102 and the upper DBR 108 so that oxidization inthe current narrowing layer 106 can be accelerated in a mesa oxidizationprocess. When the mesas shown in (A) of FIG. 1 are oxidized, oxidizedregions 106A (indicated as hatched portions in (B) of FIG. 1) oxidizedselectively are formed inward from side walls of the drive mesa 20, thecoupling portion 30 and the control mesa 40 so as to form a currentnarrowing structure. When oxidization proceeds inward at a substantiallyconstant speed, the planar shapes of non-oxidized regions 106B areformed into shapes in which the planar shapes of the mesas 20 and 40 aresubstantially reflected respectively. The non-oxidized regions 106B aresurrounded by the oxidized regions 106A. A broken line 106C in (A) ofFIG. 1 schematically designates a boundary between the oxidized regions106A and the non-oxidized regions 106B. The refractive index of theAl_(0.98)Ga_(0.02)As layer (or AlAs layer) constituting the currentnarrowing layer 106 is about 3.0. However, when the Al_(0.98)Ga_(0.02)Aslayer is oxidized, the refractive index of the Al_(0.98)Ga_(0.02)Aslayer is reduced to be about 1.7. Thus, transverse light is confined inthe non-oxidized regions 106B surrounded by the oxidized regions 106A.In addition, since the oxidized regions 106A have high electricresistance, the oxidized regions 106A substantially serve asnon-conductive regions. Carriers injected from an electrode are confinedin the non-oxidized regions 106B serving as conductive regions. Thus,the current and the light can be confined in the non-oxidized regions106B due to the current narrowing structure.

The VCSEL 10 according to the Example includes the coupling portion 30between the drive mesa 20 and the control mesa 40. The coupling portion30 couples the semiconductor layers of the lower DBR 102, the activeregion 104 and the upper DBR 108 in the drive mesa 20 with thecorresponding semiconductor layers of the lower DBR 102, the activeregion 104 and the upper DBR 108 in the control mesa 40 respectively.The coupling portion 30 includes an oxidized region 106A and anon-oxidized region 106B for coupling the oxidized region 106A and thenon-oxidized region 106B in the drive mesa 20 with the oxidized region106A and the non-oxidized region 106B in the control mesa 40respectively. Thus, the drive mesa 20 and the control mesa 40 arecoupled optically. In addition, the coupling portion 30 serves forpropagating a part of light generated by the drive mesa 20 toward thecontrol mesa 40 and feeding the light reflected on the control mesa 40back to the drive mesa 20. Accordingly, the equivalent refractive indexof the coupling portion 30 is designed to be smaller than the equivalentrefractive index of the non-oxidized region 106B in each of the drivemesa 20 and the control mesa 40. The equivalent refractive index usedherein designates an effective refractive index of semiconductormultilayer films with different refractive indices laminated verticallyon the substrate (the refractive indices of the multilayer films areregarded as the refractive index of a single layer), the effectiverefractive index being obtained by an equivalent refractive indexmethod. The “equivalent refractive index” is also called as “effectiveindex”.

In the example shown in (A) of FIG. 1, the coupling portion 30 accordingto the Example is processed into a constricted shape 32 whose oppositeside surfaces are inclined inward to be narrow in a Y direction. Sincethe current narrowing layer 106 is oxidized from the side surfaces ofthe constricted shape 32, the Y-direction width W of the non-oxidizedregion 106B in the coupling portion 30 is made narrower than theY-direction width W of the non-oxidized region 106B in each of the drivemesa 20 and the control mesa 40. In this manner, the equivalentrefractive index of the coupling portion 30 is controlled to be smallerthan the equivalent refractive index of each of the drive mesa 20 andthe control mesa 40. Incidentally, the shape and the size of thecoupling portion 30 may be selected desirably. When the shape and thesize of the coupling portion 30 are selected suitably, the Y-directionwidth of the non-oxidized region 106B in the coupling portion 30 can beactually made narrower than the Y-direction width of the non-oxidizedregion 106B in each of the drive mesa 20 and the control mesa 40 tothereby obtain a desired equivalent refractive index. Incidentally, theY-direction width of the non-oxidized region 106B in the couplingportion 30 may be zero. That is, the non-oxidized region 106B of thedrive mesa 20 and the non-oxidized region 106B of the control mesa 40may be disconnected from each other. In addition, the Y-direction widthW of the non-oxidized region 106B in the coupling portion 30 is a factorfor determining a coupling amount or coupling efficiency between thedrive mesa 20 and the control mesa 40 as will be described later.

P-side electrodes 110 and 120 made of metal and coupled to the resonatorof the drive mesa 20 and the resonator of the control mesa 40independently and respectively are formed on the upper DBR 108. Thep-side electrodes 110 and 120 are made of metal such as Au or a laminateof Au/Zn/Au, etc. The p-side electrode 110 is formed into a “V”-shape toextend along two sides of the drive mesa 20 to be thereby electricallyconnected to the upper DBR 108. In a preferable mode, the p-sideelectrode 110 is formed in a position not to overlap with thenon-oxidized region 106B. In other words, the p-side electrode 110 isformed in a region not to extend beyond the boundary 106C between theoxidized region 106A and the non-oxidized region 106B. Similarly, thep-side electrode 120 is formed into a “V”-shape to extend along twosides of the control mesa 40 to be thereby electrically connected to theupper DBR 108. The p-side electrode 120 is also formed in a region notto extend beyond the boundary 106C between the oxidized region 106A andthe non-oxidized region 106B. In addition, an n-side electrode 130shared by the drive mesa 20 and the control mesa 40 is formed on theback surface of the substrate 100.

The p-side electrode 110 is a drive electrode for driving the drive mesa20 (hereinafter the p-side electrode 110 will be referred to as driveelectrode). The other p-side electrode 120 is a control electrode forcontrolling the optical feedback of the control mesa 40 (hereinafter thep-side electrode 120 will be referred to as control electrode). In theexample in FIG. 1, the drive electrode 110 and the control electrode 120are formed on the opposite ends of the mesas 20 and 40 respectively.However, the invention is not limited thereto. For example, as long asthe VCSEL 10 according to the Example can achieve higher speed than abackground-art VCSEL, the drive electrode 110 and the control electrode120 may be formed in places other than the opposite ends.

The VCSEL 10 in the Example has a coupled resonator structure in whichthe drive mesa 20 having the drive electrode 110 formed therein and thecontrol mesa 40 having the control electrode 120 formed therein arecoupled to each other through the coupling portion 30 having theconstricted shape, as described above. Although the rectangular coupledresonators having the same shape are shown in the Example, rectangularcoupled resonators having different sizes may be used or coupledresonators having circular shapes other than the rectangular shapes maybe used. The length L1 of one side of the non-oxidized region 106Bformed in each of the drive mesa 20 and the control mesa 40 is, forexample, 8.5 μm. The length L2 of a longer axis of the non-oxidizedregions 106B formed in the coupled resonators is, for example, 28 μm.The lengths L1 and L2 of the non-oxidized regions may be made smaller orlarger than the aforementioned values in accordance with design. Whenlight in a fundamental transverse mode is emitted from the drive mesa20, the length L1 may be further reduced.

Thus, the drive mesa 20 and the control mesa 40 having the two verticalresonator structures are formed on the substrate. When a drive signal ofa forward bias is applied between the drive electrode 110 and the n-sideelectrode 130, laser light is emitted from the surface of the upper DBR108 of the drive mesa 20 vertically to the substrate. The drive signalmay be a steady signal for emitting laser light continuously or may be apulse-like signal for modulating the laser light. In addition, in theExample, it is not always necessary to apply a drive signal to thecontrol electrode 120 of the control mesa 40. In a certain mode, a drivesignal of a forward bias not smaller than a threshold which can causelaser oscillation may be applied to the control electrode 120 so thathigh-rate modulation of the VCSEL 10 can be improved more greatly. Sincethe drive signal is applied to the control electrode 120, lightpropagated through the control mesa 40 can be amplified and the phase ofthe light can be controlled. Thus, the phase of the light optically fedback from the control mesa 40 can be controlled to be a reverse phase.Due to the optical feedback in the reverse phase, the modulationfrequency of the drive mesa 20 can be further increased, as will bedescribed later.

Here, the forward bias means a positive voltage applied to the p-typesemiconductor layer inside the laser diode and a negative voltageapplied to the n-type semiconductor layer. A backward bias means anegative voltage applied to the p-type semiconductor layer inside thelaser diode and a positive voltage applied to the n-type semiconductorlayer. Incidentally, the negative voltage includes a ground potential(GND).

FIG. 2 shows the relation between the equivalent refractive index andthe light confinement distribution of the VCSEL. In FIG. 2, the stepwiseline designates the equivalent refractive index and the curve designatesthe light confinement distribution. A region D0 of a low equivalentrefractive index NL corresponds to the oxidized region 106A of the drivemesa 20. A region D1 of a high equivalent refractive index NHcorresponds to the non-oxidized region 106B of the drive mesa 20 in thesame manner as the region D0. A region D2 of the low equivalentrefractive index NL corresponds to a resonator coupling portionconstituted by the coupling portion 30. A region D3 of the highequivalent refractive index NH corresponds to the non-oxidized region106B of the control mesa 40. A region D4 of the low equivalentrefractive index NL corresponds to the oxidized region 106A of thecontrol mesa 40.

Most of light Lin generated by the resonator of the drive mesa 20 isconfined in the region D1 which is the non-oxidized region 106B.However, since the regions D0, D1, D2, D3 and D4 having high and lowequivalent refractive indices are formed continuously, the region D2 ofthe coupling portion 30 for coupling the two resonators does notcompletely confine the light Lin in the region D1 but guides a part ofthe light in the skirt of the region D1 to the control mesa 40. Evenwhen the laser light is resonated vertically in the drive mesa 20, thelaser light includes light having a slight inclination angle withrespect to the vertical direction. Therefore, a part of light in theskirt is guided to the region D3 of the high equivalent refractive indexthrough the region D2 of the low equivalent refractive index. In thestate in which light Lo guided to the control mesa 40 is confined in thecontrol mesa 40, the light Lo is propagated in a horizontal directionwhile being resonated inside the vertical resonator in a direction ofthe inclination angle. Accordingly, the propagation time of the light isslower than that of light propagated linearly in the horizontaldirection. The light propagated in the horizontal direction while beingresonated inside the vertical resonator in the direction of theinclination angle is referred to as slow light.

The slow light is propagated horizontally in the non-oxidized region106B of the control mesa 40 and then reflected on a light reflectionportion provided in an end portion of the non-oxidized region 106B ofthe control mesa 40. In the Example, the light reflection portion usesthe change of the equivalent refractive index. That is, the light isreflected on the boundary 106C between the oxidized region 106A and thenon-oxidized region 106B of the control mesa 40. The light reflected onthe control mesa 40 is fed back (optically fed back) again to theoriginal resonator of the drive mesa 20 through the coupling portion 30.Incidentally, the light reflection portion is not necessarily limited tothe one using the change of the equivalent refractive index. A separatelight reflection member may be also attached to an end portion of thecontrol mesa 40.

The slow light travels while being reflected between the lower DBR 102and the upper DBR 108. Therefore, even when a horizontal distance D3 issmall, the distance (the length of an optical path) with which the lightactually travels corresponds to several hundred times as long as thedistance D3. Accordingly, the propagation time T of the light reflectedon the light reflection portion and travelling back and forth inside theslow light portion has the same effect as if the speed of the light hasbeen delayed. In a preferable mode, the length of the optical path orthe distance D3 is adjusted so that the light incident on the lightreflection portion and the light reflected on the light reflectionportion can be set in a reverse phase relation with each other, morepreferably, in a reverse phase relation of 180° with each other.

FIG. 3 shows calculation results of frequency characteristics of theVCSEL having the background-art structure and the VCSEL according to theExample. In FIG. 3, the abscissa designates frequency (GHz) and theordinate designates modulation sensitivity (dB). Assume that the signalintensity is allowed to decrease up to −3 dB. In this case, thefrequency of the VCSEL having the background-art structure in which thecontrol mesa is not coupled is about 25 GHz whereas the frequency of theVCSEL according to the Example is about 70 GHz. Accordingly, it isproved that 3 dB frequency is improved remarkably in the VCSEL 10according to the Example, in comparison with that in the VCSEL havingthe background-art structure. The graph of FIG. 3 shows the calculationmade when the light is fed back with a reverse phase. When thesimulation in which the light is fed back with a reverse phase iscompared with another simulation in which the light is fed back with thesame phase, the result obtained in the case with the reverse phase ismore preferable than the result obtained in the case with the same phasein terms of improvement of the 3 dB frequency. Further, the bandwidth ofthe 3 dB frequency can be improved also by control of a gain based oninjection of the current from the control electrode 120. However, aneffect of improvement in the bandwidth can be obtained even in the casewhere no current is injected from the control electrode 120.

FIG. 4 shows the change of the bandwidth in the VCSEL due to thecoupling amount of the two resonators of the drive mesa and the controlmesa. The coupling amount mainly depends on the distance between thedrive mesa 20 and the control mesa 40 and the oxidization control of thecoupling portion 30. The coupling amount (coupling efficiency) tends tobe smaller when the width W of the non-oxidized region 106B in thecoupling portion 30 is larger. In FIG. 4, the bandwidth in a VCSELhaving no coupling, i.e. the bandwidth in the background-art VCSEL isindicated as the leftmost curve, and the 3 dB bandwidth is a little lessthan 10 GHz. It can be confirmed that when the coupling amount isincreased in the order of curves a, b, c, and d (that is, when the widthW of the coupling portion 30 is reduced), the bandwidth of themodulation sensitivity extends on the high frequency side. The couplingamount is a function of the width W. Accordingly, the coupling amountcan be controlled when the oxidization of the coupling portion 30 iscontrolled. That is, when the shape, the size, etc. of the couplingportion 30 are selected, a desired coupling amount can be obtained.

FIG. 5 shows the relation among the width W of the non-oxidized regionin the coupling portion between the two mesas (i.e. the drive mesa andthe control mesa), the coupling efficiency (coupling amount) and thescattering loss. As the width W of the coupling portion 30 increases,the scattering loss (broken line in FIG. 5) increases but the couplingefficiency (solid line in FIG. 5) decreases. Since the width W of thecoupling portion 30 can be controlled by the distance between the twomesas 20 and 40 and the oxidization narrowing amount of the couplingportion 30 as described above, the coupling portion 30 can be designedsuitably to obtain a desired high frequency characteristic byexperiments or simulations. Thus, the bandwidth in the VCSEL 10 can beimproved.

FIG. 6 shows the relation between the coupling amount and the 3 dBmodulation bandwidth based on the delay time of the optical feedback.The region where the coupling amount is plus corresponds to the casewhere the optical feedback is performed with the same phase, and theregion where the coupling amount is minus corresponds to the case wherethe optical feedback is performed with a reverse phase. In the VCSELhaving the background-art structure without the optical feedback, the 3dB modulation bandwidth is constant (broken line at about 11 GHz)regardless of the delay time. On the other hand, the illustratedrelation is established between the delay time and the 3 dB modulationbandwidth when the control mesa 40 is coupled to perform the opticalfeedback as in the VCSEL according to the Example. Here is shown, by wayof example, each 3 dB modulation bandwidth in which the delay time ofthe optical feedback is 1 ps, 2 ps, 2.4 ps, 3 ps, 5 ps or 10 ps. Thedelay time depends on the length of the optical path of the slow lighttravelling through the control mesa 40, that is, the delay time can becontrolled based on the length of the region D3 of the non-oxidizedregion 106B of the control mesa 40 where the slow light is propagated,as shown in FIG. 2. Particularly in the case of the reverse phasecoupling, the 3 dB modulation bandwidth is improved in any delay time.For example, when a delay of about 3 picoseconds is generated in areverse phase, the 3 dB modulation bandwidth can be improved to aboutthree times as large as that in the VCSEL having the background-artstructure.

Thus, in order to improve the 3 dB modulation bandwidth, the opticalfeedback is set preferably at a reverse phase, most preferably at areverse phase of 180°. In a first method for setting the opticalfeedback at a reverse phase, the length of the region D3 of the controlmesa 40 is controlled to select an optimal length of the optical path asdescribed above. In a second method, an optimal current is injected fromthe control electrode 120 of the control mesa 40 to thereby control thephase of light. It is a matter of course that the reverse phase of lightoptically fed back may be controlled by combination of the first andsecond methods.

Next, a second Example of the invention will be described. FIG. 7includes a schematic plan view of a VCSEL according to the secondExample and a sectional view of the same VCSEL taken along the line A-Aof the plan view. The different point from the VCSEL 10 according to thefirst Example shown in FIG. 1 is a point that the VCSEL 10A according tothe second Example has an insulation structure in which a couplingportion 30 between the drive mesa 20 and the control mesa 40 iselectrically insulated. In the drawings, the same constituents as thoseof the VCSEL shown in FIG. 1 are referred to by the same signscorrespondingly, and description thereof will be omitted.

As shown in FIG. 7, an insulated region 200 having high electricresistance is formed in the coupling portion 30 of the VCSEL 10A. In apreferable mode, a mask pattern for exposing the coupling portion 30 isformed on the upper DBR 108 by a well-known photolithography process andion injection of protons etc. is performed through the mask pattern sothat the insulated region 200 can be formed in the coupling portion 30.In a preferable mode, the energy of the ion injection is controlled sothat the entire depth of the p-type upper DBR 108 inside the couplingportion 30 can be insulated. Incidentally, it will go well as long asthe insulated region 200 is insulated at least partially on the emissionsurface side of an active region 104. In a preferable mode, theinsulated region 200 is formed all over the range of the couplingportion 30 in the Y direction. However, the insulated region 200 may beformed in a part of the range in the Y direction. In addition, the widthof the insulated region 200 in the X direction may be selecteddesirably.

In the Example, the insulated region 200 is formed in the couplingportion 30 to make a structure in which carriers are prevented frombeing injected into the coupling portion 30 from the drive electrode 110or the control electrode 120, so that the decrease of the gain can besuppressed. Further, the scattering loss can be also increased. Accuracyin controlling the optical feedback with the reverse phase can beimproved. In addition, in the Example, a certain degree of improvementof the 3 dB modulation bandwidth can be achieved even when no current isinjected from the control electrode 120. Incidentally, although theinsulated region 200 is formed by ion injection, the insulated region200 may be formed by another method than ion injection. For example, agroove may be formed in a part or the whole of the upper DBR 108 tothereby provide a function equivalent to the insulated region 200.

Next, a third Example of the invention will be described. FIG. 8includes a schematic plan view of a VCSEL 10B according to the thirdExample and a sectional view of the VCSEL 10B taken along the line A-Aof the plan view. In the VCSEL according to each of the first and secondExamples, light is also emitted from the control mesa 40 whereas theVCSEL 10B according to the third Example has a structure in which lightemission from the control mesa 40 is suppressed. In FIG. 8, constituentsthe same as those in the VCSEL shown in FIG. 7 will be referred to bythe same signs correspondingly, and description thereof will be omitted.

When light is emitted from two places in the drive mesa 20 and thecontrol mesa 40 respectively due to light emission from the control mesa40, it is difficult to control the optical mode of the emitted light.Therefore, in the VCSEL 10B according to the Example, a controlelectrode 122 is formed on the upper DBR 108 to cover the non-oxidizedregion 106B of the control mesa 40 as shown in FIG. 8 (the controlelectrode 122 is indicated as a hatched portion in order to make it easyto identify the control electrode 122 in (A) of FIG. 8). In the exampleshown in FIG. 8, the coupling portion 30 is insulated, for example, byion injection in the same manner as in the second Example. However, thecoupling portion 30 does not have to be particularly isolated as in thefirst Example.

In the Example, the light emission region is limited to one place in thedrive mesa 20. Therefore, it is easy to control the optical mode of theemitted light to thereby lead to improvement of the coupling efficiencywith an optical waveguide portion such as an optical fiber, incomparison with the case where light is emitted from two places in thedrive mesa 20 and the control mesa 40 respectively.

Incidentally, other than the structure in which the control electrodeper se covers the non-oxidized region 106B of the control mesa 40, astructure in which a material having light shielding properties againstan oscillation wavelength is added or a dielectric multilayer filmreflector etc. is added may be used as a unit for controlling the lightemitted from the control mesa 40.

FIG. 9 is a sectional view showing an example of the configuration of anoptical transmission apparatus according to the Example. The opticaltransmission apparatus 400 includes a metal stem 420 mounted with anelectronic component 410 having the VCSEL 10/10A/10B formed therein. Thestem 420 is covered with a hollow cap 430. A ball lens 440 is fixed tothe center of the cap 430. A cylindrical housing 450 is further attachedto the stem 420. An optical fiber 470 is fixed to an end portion of thehousing 450 through a ferrule 460. Laser light modulated from theelectronic component 410 is condensed by the ball lens 440. The light isincident on the optical fiber 470 and transmitted therefrom.Incidentally, any other lens than the ball lens, such as a biconvex lensor a plano-convex lens, may be used.

Although preferable Examples of the invention have been described abovein detail, the invention is not limited to the specific Examples.Various modifications or changes can be made without departing from thescope or spirit of the invention stated in Claims. Although VCSELs usingAlGaAs compound semiconductors have been exemplarily described in theExamples, another VCSEL using a group III-V compound semiconductor layermay be used alternatively.

What is claimed is:
 1. A vertical-cavity surface-emitting laser diodecomprising: a first resonator that includes a first current narrowingstructure having a first conductive region; a second resonator thatincludes a second current narrowing structure having a second conductiveregion; and a coupling portion that couples the first resonator with thesecond resonator, the coupling portion having a third conductive region,the third conductive region connecting the first conductive region andthe second conductive region, wherein: the first, second, and thirdconductive regions are formed in common current narrowing layer, a widthof the third conductive region is narrower than each of widths of thefirst and second conductive region, and an equivalent refractive indexof the coupling portion is smaller than an equivalent refractive indexof each of the first resonator and the second resonator.
 2. Thevertical-cavity surface-emitting laser diode according to claim 1,wherein the first, second, and third conductive regions are surroundedby a non-conductive region formed in the common current narrowing layer.3. The vertical-cavity surface-emitting laser diode according to claim2, wherein the non-conductive region is an oxidized region.
 4. Thevertical-cavity surface-emitting laser diode according to claim 2,wherein the first resonator has a first electrode that supplies electricpower to the first resonator, and the non-conductive region of thesecond resonator is covered with a light-shielding member.
 5. Thevertical-cavity surface-emitting laser diode according to claim 1,wherein a distance between the first resonator and the second resonatorand a width of the coupling portion are set at values with which highfrequency characteristics is improved more greatly than the firstresonator with which the second resonator is not coupled.
 6. Thevertical-cavity surface-emitting laser diode according to claim 1,wherein the first and second resonator have different sizes.
 7. Thevertical-cavity surface-emitting laser diode according to claim 1,wherein the first and second resonator have different shapes.
 8. Thevertical-cavity surface-emitting laser diode according to claim 1,wherein the first resonator has a first electrode that supplies electricpower to the first resonator, and the second resonator has a secondelectrode that controls modulation frequency of the first resonator. 9.The vertical-cavity surface-emitting laser diode according to claim 1,wherein the first resonator is driven by a first drive signal, and thesecond resonator is driven by a second drive signal different from thefirst drive signal during the first resonator being driven by the firstdrive signal.
 10. An optical transmission apparatus comprising: anelectronic component including a vertical-cavity surface-emitting laserdiode; and a housing that covers the electronic component and to whichan optical fiber is fixed, wherein the vertical-cavity surface-emittinglaser diode includes: a first resonator that includes a first currentnarrowing structure having a first conductive region, a second resonatorthat includes a second current narrowing structure having a secondconductive region, and a coupling portion that couples the firstresonator with the second resonator, the coupling portion having a thirdconductive region, the third conductive region connecting the firstconductive region and the second conductive region, wherein: the first,second, and third conductive regions are formed in common currentnarrowing layer, a width of the third conductive region is narrower thaneach of widths of the first and second conductive region, and anequivalent refractive index of the coupling portion is smaller than anequivalent refractive index of each of the first resonator and thesecond resonator.
 11. The optical transmission apparatus according toclaim 10, wherein the first resonator has a first electrode thatsupplies electric power to the first resonator, and the non-conductiveregion of the second resonator is covered with a light-shielding member.